With the increase in electronic imaging information capacity and use, a need for imaging systems capable of being addressed by a variety of electronic sources is also increasing. Examples of such imaging systems include thermal transfer, ablation (or transparentization) and ablation-transfer imaging. These imaging systems have been shown to be useful in a wide variety of applications, such as, color proofing, color filter arrays for liquid crystal display devices, printing plates, and reproduction masks.
The traditional method of recording electronic information with a thermal transfer imaging medium utilizes a thermal printhead as the energy source. The information is transmitted as electrical energy to the printhead causing a localized heating of a thermal transfer donor sheet which then transfers material corresponding to the image data to a receptor sheet. The two primary types of thermal transfer donor sheets are dye sublimation (or dye diffusion transfer) and thermal mass transfer. Representative examples of these types of imaging systems can be found in U.S. Pat. Nos. 4,839,224 and 4,822,643. The use of thermal printheads as an energy source suffers several disadvantages, such as, size limitations of the printhead, slow image recording speeds (milliseconds), limited resolution, limited addressability, and artifacts on the image from detrimental contact of the media with the printhead.
The increasing availability and use of higher output compact lasers, semi-conductor light sources, laser diodes and other radiation sources which emit in the ultraviolet, visible and particularly in the near-infrared and infrared regions of the electromagnetic spectrum, have allowed the use of these sources as viable alternatives for the thermal printhead as an energy source. The use of a radiation source such as a laser or laser diode as the imaging source is one of the primary and preferred means for transferring electronic information onto an image recording media. The use of radiation to expose the media provides higher resolution and more flexibility in format size of the final image than the traditional thermal printhead imaging systems. In addition, radiation sources such as lasers and laser diodes provide the advantage of eliminating the detrimental effects from contact of the media with the heat source. As a consequence, a need exists for media that have the ability to be efficiently exposed by these sources and have the ability to form images having high resolution and improved edge sharpness.
It is well known in the art to incorporate light-absorbing layers in thermal transfer constructions to act as light-to-heat converters, thus allowing non-contact imaging using radiation sources such as lasers and laser diodes as energy sources. Representative examples of these types of elements can be found in U.S. Pat. Nos. 5,308,737; 5,278,023; 5,256,506; and 5,156,938. The transfer layer may contain light absorbing materials such that the transfer layer itself functions as the light-to-heat conversion layer. Alternatively, the light-to-heat conversion layer may be a separate layer, for instance, a separate layer between the substrate and the transfer layer.
Constructions in which the transfer layer itself functions as the light-to-heat conversion layer may require the addition of an additive to increase the absorption of incident radiation and effect transfer to a receptor. In these cases, the presence of the absorber in the transferred image may have a detrimental effect upon the performance of the imaged object (e.g., visible absorption which reduces the optical purity of the colors in the transferred image, reduced transferred image stability, incompatibility between the absorber and other components present in the imaging layer, etc.).
Contamination of the transferred image by the light-to-heat conversion layer itself is often observed when using donor construcions having a separate light-to-heat conversion layer. In the cases where contamination of the transferred image by such unintended transfer of the light-to-heat conversion layer occurs and the light-to-heat conversion layer possesses an optical absorbance that interferes with the performance of the transferred image (e.g., transfer of a portion of a black body light-to-heat conversion layer to a color filter array or color proof), the incidental transfer of the light-to-heat conversion layer to the receptor is particularly detrimental to quality of the imaged article. Similarly, mechanical or thermal distortion of the light-to-heat conversion layer during imaging is common and negatively impacts the quality of the transferred coating.
U.S. Pat. No. 5,171,650 discloses methods and materials for thermal imaging using an "ablation-transfer" technique. The donor element used in the imaging process comprises a support, an intermediate dynamic release layer, and an ablative carrier topcoat containing a colorant. Both the dynamic release layer and the color carrier layer may contain an infrared-absorbing (light to heat conversion) dye or pigment. A colored image is produced by placing the donor element in intimate contact with a receptor and then irradiating the donor with a coherent light source in an imagewise pattern. The colored carrier layer is simultaneously released and propelled away from the dynamic release layer in the light struck areas creating a colored image on the receptor.
Co-pending U.S. application Ser. No. 07/855,799 filed Mar. 23, 1992 discloses ablative imaging elements comprising a substrate coated on a portion thereof with an energy sensitive layer comprising a glycidyl azide polymer in combination with a radiation absorber. Demonstrated imaging sources included infrared, visible, and ultraviolet lasers. Solid state lasers were disclosed as exposure sources, although laser diodes were not specifically mentioned. This application is primarily concerned with the formation of relief printing plates and lithographic plates by ablation of the energy sensitive layer. No specific mention of utility for thermal mass transfer was made.
U.S. Pat. No. 5,308,737 discloses the use of black metal layers on polymeric substrates with gas-producing polymer layers which generate relatively high volumes of gas when irradiated. The black metal (e.g., black aluminum) absorbs the radiation efficiently and converts it to heat for the gas-generating materials. It is observed in the examples that in some cases the black metal was eliminated from the substrate, leaving a positive image on the substrate.
U.S. Pat. No. 5,278,023 discloses laser-addressable thermal transfer materials for producing color proofs, printing plates, films, printed circuit boards, and other media. The materials contain a substrate coated thereon with a propellant layer wherein the propellant layer contains a material capable of producing nitrogen (N.sub.2) gas at a temperature of preferably less than about 300.degree. C.; a radiation absorber; and a thermal mass transfer material. The thermal mass transfer material may be incorporated into the propellant layer or in an additional layer coated onto the propellant layer. The radiation absorber may be employed in one of the above-disclosed layers or in a separate layer in order to achieve localized heating with an electromagnetic energy source, such as a laser. Upon laser induced heating, the transfer material is propelled to the receptor by the rapid expansion of gas. The thermal mass transfer material may contain, for example, pigments, toner particles, resins, metal particles, monomers, polymers, dyes, or combinations thereof. Also disclosed is a process for forming an image as well as an imaged article made thereby.
Laser-induced mass transfer processes have the advantage of very short heating times (nanoseconds to microseconds); whereas, the conventional thermal mass transfer methods are relatively slow due to the longer dwell times (milliseconds) required to heat the printhead and transfer the heat to the donor. The transferred images generated under laser-induced ablation imaging conditions are often fragmented (being propelled from the surface as particulates or fragments). The images from thermal melt stick transfer systems tend to show deformities on the surface of the transferred material. Therefore, there is a need for a thermal transfer system that takes advantage of the speed and efficiency of laser addressable systems without sacrificing image quality or resolution.